Which glass shader is physically correct?
Technically, using Maximum is better than using Add - since with Maximum the result will be in the correct range of 0.0 to 1.0 for the Factor input of the Mix node. With Add you end up in the range 0.0 to 3.0 since you’re adding three 0.0 to 1.0 values together. While this won’t in itself cause a problem (since Factor of the Mix will auto clip to 0.0 to 1.0) it could cause a problem or unexpected results if you later use the same value for other uses, not realising it’s outside range.

Without knowing what’s outside the window it’s difficult to tell which is ‘right’ - try rendering without the glass (either hide the window pane mesh or set to 100% transparent) - the thin sheet of glass should cause hardly any displacement (since it’s two parallel surfaces) - whichever more closely matches the view without the glass is by far the most accurate..

Also related : https://blender.stackexchange.com/a/90526/29586 and https://blender.stackexchange.com/a/77806/29586



After setting up a test, the second example  is by far more realistic for the refraction :

Bad refraction :


Good refraction :


This can be made even more apparent by simply rotating the glass along the Z axis.